What flashing kit do I need for a VELUX?

A VELUX window arrives as a bare unit. The flashing is the metal apron and side sections that seal it into the roof, and it is bought separately because the shape has to suit the covering it meets. Get the covering right and the water runs off the way the profile is designed to carry it. Get it wrong and you have created a channel where there should be a barrier.

The two you will meet most often are these:

• EDW is for profiled roofing materials up to 120mm deep. That covers most concrete and clay interlocking tiles, the pantiles and the deep-profile tiles you see across the 1930s semi belt and the post-war estates. The apron is pressed with a pleated skirt that folds down over the contours of the tile.
• EDL is for slate and other flat coverings up to about 8mm thick. It sits lower and flatter, which suits natural slate, fibre-cement slate and thin flat tiles. Use it where the roof surface is close to level rather than rolling.

Then there are the recessed and specialist versions. EDJ and EDP drop the window deeper into the roof for a flush look against plain tiles or slate, which some conservation settings ask for. EDN is the recessed option for flat roofing materials. If your covering is unusual, a standing-seam metal roof or a thick stone slate, there is a specific flashing for it, and guessing is the wrong approach. Read the covering first, then choose.

Every VELUX window carries a size code, a letter and a number together, such as CK04, MK06, UK08 or SK06. That code is the frame’s exact width and height. The flashing kit is manufactured to the same code, so a CK04 window takes a CK04 flashing and nothing else. There is no adjustable, one-size flashing.

The code is on the data plate, which sits on the top of the sash and is visible when you open the window and look up at the frame. If you are replacing a failed unit rather than cutting a new hole, the simplest path is to read the old code and order the like-for-like size, because the opening and the timber around it are already built to it. If the plate has weathered off, the frame’s outer dimensions give you the code from any VELUX size chart. Order the flashing and the window in the same code at the same time and the parts meet as designed.

A single window takes a single flashing kit. The moment you want two or more in one roof, the flashing changes, because the gap between the frames has to be sealed as a group rather than as separate units with a weak seam down the middle.

For grouped windows VELUX supplies combination flashings, EKW for profiled tiles and EKL for slate, which handle windows set side by side, stacked one above another, or arranged in a block of three or four. These fix the spacing between the frames, so the timber has to be built to match the flashing rather than the other way round. If you are planning a run of rooflights over a deep room to pull daylight into the middle of it, decide the layout before the timberwork goes in, because the combination kit dictates the centres. This is common on the rear kitchen extensions we fit across south Essex, and it is worth getting on paper early. Our VELUX window installation service sizes the group and the flashing together so the roof structure is set out for it from the start.

A flashing kit is the weathering set: the head, sill and side sections that seal the window to the covering. That is what keeps the rain out at the surface. Underneath it, a proper installation wants two more parts that are sold separately and are easy to overlook.

• BFX is the underfelt collar with a moulded drainage gutter. It joins the window to the roofing membrane below the tiles and carries any water that gets past the surface down and out. Skip it and you are relying on the top flashing alone, which is the layer most exposed to wind-driven rain.
• BDX is the insulation collar that frames the unit and closes the thermal gap between the window and the rafters. It matters for the Part L thermal performance of the finished job and for stopping the cold bridge that shows up as condensation around a poorly fitted frame.

None of that changes the glass. The flashing and collars seal and insulate the hole; they do nothing about how much summer heat the glazing itself lets through. That is a separate specification, and on a warm roof it is the one that decides whether the room is usable in July.

Two things sit alongside the flashing choice and are worth settling in the same conversation. The first is roof pitch. VELUX centre-pivot and top-hung windows are made for pitched roofs, typically from about 15 degrees up to 90, with some able to go down towards 10 degrees on the right underfelt arrangement. Below that you are into flat-roof territory and a different product entirely. If your roof is genuinely low or flat, a standard VELUX and its flashing are not the answer, and a pitched-roof skylight installation is where the compass-facing decisions start.

The second is orientation, and here the flashing is the least of it. Southend-on-Sea faces south across the Thames Estuary, so a great many rear roofs point straight at the sun for most of the cooling season. The flashing kit is identical whether the window faces north or south. The glass should not be. A south-facing VELUX over a kitchen wants solar-control glazing with a low G-value, around 0.3 or below, so it does not turn the room into a greenhouse, while a north-facing one is fine on standard glass and will give you useful winter warmth for nothing. Order the correct flashing for the covering, then choose the glass for the direction the roof faces. Our energy-efficient skylight installation work is built on exactly that split.

So, to settle it: read your covering to pick EDW, EDL or a recessed profile; read the size code off the window and order the flashing to match; add a BFX underfelt collar and a BDX insulation collar; and use a combination flashing if there is more than one window in the roof. If you would rather hand the whole specification over, tell us the covering, the roof pitch and which way the roof faces, and we will come back with the right kit and the right glass. Ring 01702 898232 or request a quote and we will look at the whole roof, not just the window in it.